From what I understand, they are fine with forks of Signal as long as these forks have distinct branding and don't depend on OWS services to operate, I'd imagine because they don't want the operational burden of ensuring the compatibility of third party clients nor confused users going to OWS for support instead of the third party devs.

Which, imo, is a pretty reasonable decision.

I think I liked Signal better when it was just encrypting text and sending it over SMS. I noticed recently that someone has been maintaining such a fork, under the name "Silence", but haven't had a chance to try it yet.

I used Silence as my primary SMS app for several years while I was on Android. Overall I had no complaints. Beware though, they seem to have been booted out of the Play Store at some point, so you might need to install through F-Droid.

They don't block third party clients ala WhatsApp, but going on GitHub and arguing with people isn't a good thing either :/