I try to answer your questions but one thing beforehand. Micha mentioned a change in the process of e-mail-address validation. This change from a regular expression to the function
Yes, I did try to enter e-mails of few domains (gmail, yahoo, rapideuphoria311.com).
All services use the latin alphabet and ".com" als top level domain (TLD) so this should be no problem.
All e-mails, including @rapideuphoria311.com are using only (7-bit) ASCII characters (I assume that digits 0-9 are included...).
Also that should be no problem.
Are there any restrictions of length of passwords, database name, etc?
There are no restrictions for the database connection related data from the side of the script. If there would be restrictions these restrictions would be originated in the database server in itself (like i.e. the maximal length of table names).
Are the characters '_'; '-'; '0-9'; 'a-z'; 'A-Z' can be used for all purposes?
Yes, these chars can be used everywhere.
In the "phpMyAdmin" there is a setting for "Server connection collation": 'utf8mb4_unicode_ci'
No problem. Our script resets the connection collation to "utf8_general_ci". because of that the default value does not come into the play.
The database name is: 'rapideup_mylittleforum-2.4.20'
Database user name has '_' in it.
No problem at all.
But nothing of that has anything to do with the e-mail-address validation.
Trenne niemals Müll, denn er hat nur eine Silbe!