I don't have a fediverse account (yet), so excuse me for replying via a
different medium. I think this is a great idea! If someone wants to
host proprietary software, they can run their own instance ;).
As an alternative option, you could have special plans that allow
closed-source projects for twice the price of the equivalent FOSS-only
pricing tier. Then donate the difference to open source development, be
it subsidies for the OSS-only plans, bounties on sourcehut issues, your
liberapay/patreon (this one might seem a little less "charitable"), or
directly to the FSF, EFF, SFC, etc.
The way I understand your current pricing scheme, you'd probably want
to limit closed-source users to the highest tier ($20/month instead of
$10/month) so there's a substantial difference between the "FOSS price"
and the non-FOSS price.
If I were in your shoes (and didn't care about profitability at all), I
would've AGPL-d the whole platform and maybe even hacked in a license
clause applying the GPL to anything built on top of it (essentially the
opposite of the Linux kernel's syscall note), but I totally get why
that isn't a viable option for any project that actually wants users...