I'm not aware of any legitmate fork that supports the RT-AC87U. Keep in mind that Asus only releases 382 firmwares for that model, so it's no longer possible to keep it in sync with newer 384 or 386 code. Best one could do is partial backports, similar to what John does for previous models - and in his case it's a bit easier to do because that older code has far less closed source pieces than 382 does.
It's unlikely there will ever be a fork for the RT-AC87U so it's worth starting looking at alternatives.
My plan is to go for a pfSense router running on a mini PC from Aliexpress (the most popular brand is called Qotom) and reuse my RT-AC87U as an AP as its wifi is going to be good for a few more years.
Then when there's a good choice of wifi 6E APs I'll get one of those. Meantime running pfSense you're going to have your hardware supported for many years as it's fully open source software.