Many of us use own IMAP servers or an authenticated secure method to connect to a network with an IMAP server. A decision to not to provide an option to the user to have a connection to the IMAP server without security may have an educational purpose but serious people perhaps do not need to be educated by developers of Vivaldi. I think a missing option for defining access to an IMAP/SMTP server with NO security is a serious drawback for Vivaldi. Not because no security might sometimes still be safe but because the developers of Vivaldi here make a decision instead of the user, which in a world outside Microsoft Windows not the developers but the user should make alone.
And yes, no security may still be safe.
@Doelli You can edit the color of the specific calendar, but not the color of each individual entry. As far as I know Vivaldi uses Roundcube and I think it’s unlikely Vivaldi will do anything about the existing feature set. You would probably have to ask Roundcube about it.