I'm trying to think when the last time an anime was outright canceled due to production issues (iirc the Yuri on ice movie was first announced almost 7 years ago and had a release date of around 2018-2019, but then got pushed back to 2020, then radio silence until this cancelation announcement)
I vaguely recall hearing about an outlaw star sequel that got abandoned from 20 years ago
Kon had some projects like dream machine but that got shelved because of his death and no director can mimic his style
Sunrise had an sci fi original called mirai arise that was supposed to air in 2016
Gianax had a few things in the works but ever since Anno left and took half the noteworthy staff and Imaishi the other half, they have done almost nothing in almost a decade
I do also recall some isekai anime that got cancelled before it aired because of some comments the author made on twitter about gooks and chinks, but this was back in 2017-2018, so it wasn't the canary in the coal mine of the death of anime as some threads on /a/ where doomsaying
Here are some other cancelled anime:
Saint of Braves Baan Gaan -- cancelled due to the poor performance of Brave toys. Reworked into Brave Saga and GEAR Fighter Dendoh.
Photogrizer -- a ninth Brave series, cancelled for the same reason as Baan Gaan. Will appear in the Sworgrader manga.
Kanzen Shouri Daiteioh -- a fourth Eldran series that would have tied together the previous three shows. A pilot and a manga were made.
Sunrise Zeorymer sequel -- would have been made in the late 90s; was reworked into the Zeorymer Omega manga.
Dancouga Burn -- the intended second anime in Ashi Pro's intended mecha reboot series (the first of which was VS Knight Lamune) to capitalize on SRW; a manga was made, however.
Goshogun remake -- the third anime intended for Ashi Pro's mecha reboot series.
Dendoh sequel -- would have featured a new cast of children fighting plant monsters led by a space pirate, as well as new GEARs. Hokuto, Ginga, and Eris would have been all grown up, and Alktos would have become an ally of the Earth similar to Macross's human/Zentradi alliance. Cancelled due to the first show's poor toy sales, but Vega's planned character design was used in My-Hime (fun fact: My-Hime was inspired by Dendoh's C-DRiVE idol group).
Bubblegum Crisis Tokyo 2041 -- would have been funded by ADV Films, but was cancelled because the staff of 2040 were too busy. It is speculated that ADV imported Parasite Dolls (which was planned to be a full TV show but only had two episodes finished, likely due to the poor performance of AD Police) and commissioned a third episode to complete the OVA.
Justeen -- a planned AIC mecha franchise that would have featured famous plamo mecha. The first episode was released as an Oreimo tie-in, but no further episodes were made.
Mahoroba -- a movie adaptation of a Leiji Matsumoto manga. There was another cancelled Leiji Matsumoto anime that would have featured a transforming Yamato, but only the gacha game was made.
