I've been arbitrarily hit by this stupid thing too, after chatting for years on a 0 violations, and even paying account, too. The whole concept makes no sense (heck, the Twitch website doesn't even allow me to chat in my OWN channel, let alone in channels I am Mod/VIP) and has no benefit to the platform EXCEPT more personal data harvesting. It's repugnant behavior because of this, as it only hurts and hinders legitimate users while it does not hinder bad actors (worse, SMS verification is known to be notoriously insecure). Some users aren't even ABLE to phone verify, because they don't have a phone number, or their number is unsupported, or other reasons. But, ofc, it sadly seems doubtful that Twitch cares about any of these things, or about users' privacy, security and well-being... Twitch benefits from collecting phone numbers to potentially use in data analytics, user tracking and profiling, and global personal data sharing/selling to other companies - so, unfortunately, that appears to be all they care about regarding this... What is certain is that they have ignored this issue for years. For the reasons below,, phone verification shouldn't even be a thing at all on the platform, and should be completely replaced with something that is both actually effective for its purpose and isn't user-hostile. Paying accounts should be auto-verified. Simply having a user solve a series of varied different captchas and/or a quiz once to verify as a human is already better than having the user phone verify once. For increased mitigation, these challenges could be changed periodically if desired, in case bots manage to start to adapt to them and pass them as well.

Bad actors including bot accounts get around a phone number requirement routinely, since they can freely and without compunctions use phone numbers that are temporary, disposable, stolen, hijacked or provided by companies on the internet providing free or paid (with a few cents) phone number renting services or SMS verification services. No bad actor or bots dev will use their own personal phone number, while they will get around this requirement. Meanwhile, the legitimate user gets screwed. And if they give out their personal phone number, it likely gets shared, sold, added to tracking profiles, and publicly revealed n the next Twitch hack or data leak (or if any rogue Twitch employee or visitor accesses the data at any time), and subsequently potentially used against them and their personal safety. Once info is out there, you can't take it back, and a personal phone number of someone is very sensitive information that can quickly lead to potentially finding their real name, home address, place of work, family, etc (not to mention that anyone with the number automatically can call them, sign up for services with their phone number, potentially implicate them in things or impersonate them, hijack their phone number or incoming messages, etc). Dangers like doxxing, stalking, harassment and SWATting are already known dangers on both the internet at large and on Twitch specifically. Even cybersecurity companies get hacked and get their data stolen all the time, not just Twitch, so even if they improved their security, there is no guarantee of safety for the users once the personal information is stored in Twitch's database. They're screwed and potentially in future danger, whether they are aware of it or not.

It's worth mentioning that Twitch doing this randomly without due reason or explanation to select users isn't just morally corrupt, it's very likely to not even be legal. There are unfortunately clear elements of undue discrimination as well as fraud for paid services (in cases where affected users have, for example, any subs which have been paid for, but they now cannot make use of or enjoy due to have been arbitrarily blocked from this central part of the platform and its main point (interaction) - which any new user/account can in fact enjoy freely...)

I could also mention this user-hostile "feature" is buggy or behaves inconsistently in some ways, but it doesn't really matter (well, one of the inconsistencies is that chatting while giving bits IS allowed, but that's financially beneficial to Twitch). Even if this were completely unintended and an accidental bug from the get go, Twitch is showing no signs of desiring to change it for years. I had also contacted Twitch Support but only received a few pre-written, copy-pasted responses a few times that completely ignored everything I had written, and then the ticket was closed.
Intended or not, it would outwardly seem that both Twitch Support and Uservoice are merely "honeypots", designed to draw in user complaints and feedback to isolate them there and prevent them from "spilling" elsewhere, without any intent of addressing them. There are no indications that any human from Twitch is even reading either the Twitch Support cases or Uservoice. Very unfortunate, and for shame.

It is because of stalkers and banned user. EMail verifications can be faked easly with fake emails, but phone not.
If people who get banned and stalker would stop being annoying, then there wouldn't be any need. Many people have the phone number verification for logging in from different brwosers/apps anyway, so it doesn't change much.