I know a lot of people use Freenom domains (.cf /.ga / .gq / .ml / .tk) for their Repls and I have previously pointed people to Freenom if they cannot afford a paid domain.
I would strongly recommend that those who use these domains look elsewhere, as I believe Freenom have started shutting down due to the recent lawsuits against Freenom from Meta and the issues with the .mil domains being mistaken with .ml domains.
I had a number of low-traffic domains from Freenom and recently received several emails from Cloudflare that some domains no longer point to their nameservers.
I know that Freenom has a history of taking domains away from people, but I have not had any issues myself until this point.
The next best alternative for free domains (well subdomains) would be eu.org, but I have found their approval process to be quite slow (likely due to the amount of spammers they attract).
I personally wouldn’t trust anyone offering a free subdomain service either, as they might forget to renew and reserve the right to terminate your subdomain at any time, just like Freenom and eu.org.