The API is still fully functional for moderator accounts, including NSFW posts if you're moderating an NSFW subreddit. So, you can create a subreddit, mark it as NSFW, set it to private if you wish, and have a fully functioning Boost app again. This trick probably won't work forever, but enjoy it while it lasts.
I personally found Boost to be faster when using my own API key with ReVanced, so that might be the better option(unless being a mod somehow speeds it up)
Possibly just on my end, before the API changes Boost started to get really slow, then after patching it with ReVanced it was 10x faster, though it could also be the fact that I reinstalled Boost when patching it.