you can also just limit the amount of sign ups from one IP each day. There's more simple heuristics to prevent unsophisticated abuse like that
For lots of these countries their total allocation of IPv4 addresses is < 20 per 1000 people and the nature of their access (through glorified internet cafes) mean that you will have some IP addresses that really are totally legit, yet have LOTS of users.
One size fits all is very dangerous on the Internet.
One the one hand, I assume bad due to cheap equipment. On the other, it's not like v6 addresses are expensive and you need some way of addressing every subscriber anyway. As more people sign up (as the country gets more people with internet access), you need more equipment which could support v6 out of the box, and the excuse for CGNAT I've always heard is old equipment that is harder to upgrade than to put a NAT router in front of. Could go either way from my POV.
If the roll-out is good, then all those people are already taken care of and the minority left on v4 CGNAT aren't bothered by the collective rate limit.
(To preempt the eventual remark that users can generate a billion addresses in v6: rate limiting on v6 works by limiting whatever prefix the ISP gives out to subscribers, like /56, not individual addresses the way it's often done with v4.)
As an aside, it should also be kept in mind that not every use case involves signing entire countries up for their service, even in an ideal case.
In fiber some do the same, although thankfuly most place v4 behind a cgnat while offering ipv6.
The whole 1 ip 1 user even if dynamic quite false and is a mess.
If you're collecting sign-up data for something local, then most likely not.
FWIW, I learnt about this the hard way.
This is a classic example of how "just do this" kind of thinking can lead to terrible results.
Do you now see how "just limiting the sign ups from one IP each day" can go very very wrong?
one per library per day...