Nintendo Directs are a series of online video presentations made by Nintendo at irregular intervals to demonstrate and announce new games. Alongside other games, announcements about Super Smash Bros. 4 and Super Smash Bros. Ultimate have been made at Nintendo Direct presentations. There are some directs that were completely Smash-oriented, which are listed below.
Standard Nintendo Directs
Below is a list of announcements or mentionings of Smash Bros. in directs pertaining to other Nintendo-related subjects.
Super Smash Bros. for 3DS/Wii U
Super Smash Bros. Ultimate
Developer Direct - Super Smash Bros. for Nintendo 3DS and Wii U (June 11, 2013)
Released as a follow-up to the earlier June 11th, 2013 E3 Direct presentation, this video was Super Smash Bros. series director Masahiro Sakurai's first appearance in a Nintendo Direct. In it, Sakurai briefly summarized the aesthetics and design goals of both versions of Super Smash Bros. 4, then gave new details on several fighters that had been revealed that day. These included veterans Samus and Pit as well as newcomers Villager, Mega Man, and Wii Fit Trainer. In conclusion, Sakurai announced the opening of the Director's Room Miiverse community, where he would provide daily updates about the games' development.
Original Japanese version: https://www.youtube.com/watch?v=IvLiMHhW_5c
Several Super Smash Bros. for Nintendo 3DS-exclusive stages were seen for the first time during the showcase, but were not named or discussed.
Super Smash Bros. Direct (April 8, 2014)
On April 8th, 2014, this special Nintendo Direct was fronted by Masahiro Sakurai and was completely focused on giving information about Super Smash Bros. 4, in addition to revealing new and returning elements. It was known as the Dairantou Smash Brothers Direct, "大乱闘スマッシュブラザーズ Direct" in Japan, though this Nintendo Direct was referred to as Super Smash Bros. Direct in Japan as well. 
Original Japanese version: https://www.youtube.com/watch?v=ir98bJkma_4
Several stages were revealed and/or showcased, including past stages from Brawl and Melee. It has also been revealed that the Wii U and 3DS versions will have different past stages. In addition, it was confirmed that some stages will have boss characters that appear and attack players during normal gameplay, that can be fought and defeated; when defeated, they will erupt in a massive explosion, that will harm all players except for the one who dealt the finishing blow. It was also shown that most stages will have a "Final Destination form", which turns the stage into a single large flat platform with no additional platforms while removing all stage hazards.
The Online mode was confirmed to be present in both versions. The other multiplayer announcements revealed were:
Super Smash Bros. for Wii U: 50-Fact Extravaganza (October 23, 2014)
A Direct that revealed over fifty new elements included in Super Smash Bros. for Wii U, which aired on October 23rd, 2014 at 3 pm PDT. In the English version, it was narrated by Xander Mobus, the English announcer of Super Smash Bros. 4. The Japanese version (大乱闘スマッシュブラザーズ for Wii Uがスゴい５０の理由, Great Fray Smash Brothers for Wii U: 50 Great Reasons) was narrated by Tomokazu Sugita, the voice actor for Chrom and Takamaru.
A number of stages were confirmed for the Wii U version, including an official name for the previously leaked minecart stage. It also confirmed "big stages", designed for eight-player battles, including Big Battlefield and the aforementioned stage, as well as a Miiverse stage set for release post-launch.
Here is a list of all the stages that were fully revealed:
The mode Special Orders, which was previously teased by Sakurai on Miiverse, was officially announced. Additionally, the board game-like mode Smash Tour and Stage Builder, which were previously leaked by Amazon, were also confirmed, alongside Events, Special Smash, and other modes.
Super Smash Bros. for Nintendo 3DS / Wii U - New Content Approaching (June 14, 2015)
On June 14th, 2015 at 7:40 am PDT, a Super Smash Bros. livestream was hosted by Masahiro Sakurai, confirming two new characters, four new stages, new amiibo, the 2nd Wave of Mii Fighter costumes and the upcoming Tourney mode.
Original Japanese version: https://www.youtube.com/watch?v=oJ4QX-UIO-s
Mii Fighter costume announcements
Super Smash Bros. for Nintendo 3DS and Wii U - Final Video Presentation (December 15, 2015)
On December 15th, 2015, at 2 PM PDT, the final Super Smash Bros. 4 presentation was hosted by Masahiro Sakurai. It provided information on the previously-announced Cloud character and Midgar stage, as well as numerous new features.
Original Japanese version: https://www.youtube.com/watch?v=3bFyg4kaVE0
Mii Fighter costumes
Super Smash Bros. Ultimate Direct (August 8, 2018)
Original Japanese version: https://www.youtube.com/watch?v=OYf16WgNPjs
Simplified Chinese version: https://www.youtube.com/watch?v=ucNcI0-0XyI
Traditional Chinese version: https://www.youtube.com/watch?v=AWG3ECrnhog
Super Smash Bros. Ultimate Direct (November 1, 2018)
On October 30th, 2018, Nintendo of America announced a Nintendo Direct for Super Smash Bros. Ultimate to air on November 1st at 7:00 AM PDT / 10:00 AM EDT. Nintendo stated this would be the final Nintendo Direct presentation focused on Ultimate before the game's launch on December 7th. Series director Masahiro Sakurai once again was the host of the roughly 40-minute presentation.
Original Japanese version: https://www.youtube.com/watch?v=JBqz5vGmMAQ
Mii Fighter costumes