In our latest release v4.01 we have added a setting and the logic, that if a client is booked to a class where the class is already past, it will check for available recording file of that Zoom meeting and if available it will send the recording link email to the post signed up client. This is in conjunction with a new setting called "Send post Zoom recording" that has to be enabled.
This way you can post sign up a client to a past class. If you have recorded your meeting to Zoom cloud it will then send the recording link email to that signed up client.
Also, due to the new setting "Post email notification timing hrs" it will not send anymore a client email with the Zoom join link since the meeting is past. See this article for more information.