Speaking for myself.

I'm certainly no fan of CLAs and would much rather we adopted the Developer Certificate of Origin.

Though it's not the worst CLA I've seen.

This is troubling me because all previous contributors probably didn't sign any CLA, which means that the source code is owned by them too.

Yes I presume so, though of course IANAL.

What will happen to nginx in the future, are you planning a rug pull?

This would certainly be a lot more concerning if nginx was under a copyleft license like the GPL. But seeing as nginx is under a BSD license, F5 (or indeed anyone) can already do what they want with the code including making it proprietary, which in fact they already do with nginx plus.

Of course, like any software, if it goes from open source to proprietary the previous open source versions remain open source. There are of course already at least a couple of nginx forks such as Angie and freenginx if things ever did go truly south.

Hello,

I noticed that #908 added a CLA requirement in the CI/CD and that contributors are starting to sign it (even when their changes are not even legally significant).

This is troubling me because all previous contributors probably didn't sign any CLA, which means that the source code is owned by them too.

You should probably ask F5 official representatives why the change.
I'm not delegated to comment on this.

Regardless, see https://nginx.org/en/docs/contributing_changes.html#license