For me the library has been good enough for my own use for a very very long time. I mostly neglect it unless there's some critical issue (which there hasn't been for a very long time — but clearly infosec world knows this lib exists as when I do make changes with crap commit messages I get emails asking what the implications are). I don't improve it at all as my time is better spent on my day job.
I've often thought that there's room for improvement such as a DOM style sanitizer to validate HTML input rather than just a SAX style sanitizer, perhaps formatting of output in addition to sanitising input, transformation rules to allow a safe embedly type thing, etc.
When I got the initial donation I was surprised, first ever bit of support for open source software I'd written (as this was not written on company dime).
Even at $10 per month it is motivating enough to think someone values it. If it accrues into something significant then I may actually feel motivated to improve it rather than just support it.
Interesting is that I'd regard this OSS lib as successful by usage as sourcegraph says 803 projects use it — but given that a fair number of those are things like Hugo which in turn have thousands of forks and many more thousands of instances of use, well it does appear that this sanitizer is to safe HTML in Go what https://xkcd.com/2347/ illustrates. Originally it was written for my own use and it's now used by virtually everything in the Go world that makes a website.
Perhaps people don't know this and libraries like it exists though? Perhaps they import some web framework and this came with it? Well, for that awareness thank you to thanks.dev
Of the models I've seen so far for supporting individuals who create OSS code this one stands out by highlighting dependencies. Likely the solution is a blend of things... a commission type system for rare engineers and skills (i.e. https://words.filippo.io/pay-maintainers/ ) and thanks.dev for the long-tail of those who have done things and you want to use it long-term. There are also companies who create OSS software, and if you value their work and don't want what they produce to go behind Enterprise differentiation then perhaps pay for their services too.