Another update:
Apple has removed from Apple's 'Mail User Guide' in the ׳Tips׳ app all the information about the option to go offline in an email account
(The option that was listed at the beginning: "option to change a mailbox to offline mode has moved to be under 'Mailbox' > 'Availability Status' > 'Take [account] offline'" is no longer exists and there is no mention or reference to it)
In fact, from another conversation I had with Apple regarding my case - there is no option in 'Tahoe' to switch to offline mode in an email account.
The only option in 'Tahoe' as it appears on the website: Change Account Information settings in Mail on Mac - Apple Support (IL), is to turn off the account (not to delete it, but only to turn it off) and in fact this disconnects the same email account so that it does not appear in the mail application until the next time the account is reactivated.
The description of this according to Apple:
Enable this account = Make an account active or inactive. When you deselect the checkbox for an account (make it inactive), the account is disabled and taken offline—its mailboxes and messages don't appear in Mail.
I don't understand why Apple completely removed in 'Tahoe' the option to take email accounts offline and instead only gave users the option to turn off the email inbox.
This option of turning off the email box is bad:
1. It is complicated to operate through the settings.
2. It does not allow access to e-mails but actually completely turns off the e-mail box so that it does not appear at all in the e-mail application until the e-mail box is turned on again.
(On 'Sequoia' in offline mode there was at least access to emails, now on 'Tahoe' this is not possible.)
In the conversation I had with Apple in this case, I was told that maybe Apple will try to improve turning off the email box through the settings so that it will be easier.
Beyond that, there is no reference from Apple on whether there is a possibility that it will bring back in the future the possibility to make a mailbox offline like it was in 'Sequoia'.
It's a shame that there was no reference to this issue from Apple in the list of changes in the software update to 'Tahoe'.
I'm sure there are many more users who work with several email accounts through Apple's email app and will now think about whether to connect those accounts to Apple's email app or download separate apps for those email accounts because of this issue that you can no longer go offline mode in 'Tahoe'.
I hope this case will reach more people who will contact Apple about it and maybe they will understand the importance of offline mode in email accounts in its app and maybe they will change their decision and return it in 'Tahoe' as it was in 'Sequoia'.