SMTP will not be disabled or can be enabled again after the cutoff date:

After checking with Microsoft, here’s what will happen:

• If a tenant has never used SMTP AUTH, Microsoft is already actively blocking the protocol (see my earlier article) by setting the SmtpClientAuthenticationDisabled organization-wide control. Tenant administrators can disable SMTP AUTH for the organization today by running the command:

• If a tenant is using SMTP AUTH, Microsoft will not disable the protocol. The presumption is that the organization knows how they use SMTP AUTH and has good business reasons to continue using SMTP AUTH.
• If a tenant discovers that they need to use SMTP AUTH after Microsoft disables the protocol, they can run the Set-TransportConfig cmdlet to update SmtpClientAuthenticationDisabled to $False. However, the big downside in taking this step is that it enables SMTP AUTH across the entire tenant. A per-mailbox setting is available to allow access to SMTP AUTH that overrides the organization configuration. It’s obviously better to limit access to potentially insecure protocols, so it’s recommended that you enable the protocol on a per-mailbox basis to restrict access just to the mailboxes which need to use SMTP AUTH. For example, this command allows the James Smith mailbox to use SMTP AUTH:

https://office365itpros.com/2021/09/27/smtp-auth-exception/

Also, an additional important point, each tenant can request an extension to Jan 1.  Do so at your own peril as it would stink to have to work New Years day if things stop.  https://admin.microsoft.com/AdminPortal/?searchSolutions=Diag:%20Enable%20Basic%20Auth%20in%20EXO#/homepage

None of this affects on-premise or non MS Hosted Exchnage.