I have the same issue! Considering the number of users who use Gmail, which I would bet is one of the top if not the top email service in the country to not be able to create rules is for me non-starter. If I had known about this issue, I would have never started using Outlook! Sure, they say you can convert back to the classic outlook but during research it's my understanding that rules for Gmail will not work in the classic version either and it will only convert the main account back to the classic version. Any other accounts will be lost. I personally am looking for an email client that will allow rules for both Gmail and Outlook accounts.
Additionally, several days ago, there was a huge article which discussed the hazards of clicking on unsubscribe in some of the emails you receive and how dangerous that can be. It could very well be a bad actor and clicking on unsubscribe could release malicious viruses, malware, and ransomware on your device. So, clicking on unsubscribe to try to reduce the number of emails received is not really an option. The number one recommendation to reduce the number of emails, spam and otherwise is to use rules to sort them out by sender into separate folders and then delete the whole bunch all at once without opening. Before you say, "Well you can sort by sender and then delete within the inbox" I tried that and for some reason that does not work either, yes you can sort by sender but unlike older versions of Outlook where you could select the sender and delete the whole batch of emails with one click you actually have to select each email or click the first email of the batch and then scroll down to the last email from that sender and then select shift key and click the last email to select and then delete. If anyone knows how to actually sort and delete everything from a sender with one click like you used to be able to do, that would be very helpful and could be an option until MS adds rule capability for Gmail, but I've been waiting for at least a year or more so.....