Jeremy mentions that his feed reader of choice, NetNewsWire, will try to find an RSS feed for many email newsletters. (Mine, Reeder, does something similar.) A lot of the newer newsletter services—Substack and Buttondown, for instance—provide this.
Some other services (often older providers focused on email delivery infrastructure) don't, so rather than denying myself the pleasures of writers who choose to use these systems, I turn to Kill The Newsletter!, one of those small single-purpose tools that does a job really really well: in this case, creating mailboxes and generating RSS feeds from anything that gets sent to them!
I know there are some feed readers that give you an email address to forward newsletters from your primary email (there are a couple of providers for whom I have to do this, e.g. the FT or Bloomberg, where my accounts are tied to my email address), but I really like having everything split out—if I ever want to unsubscribe to something I can do it as I would to a feed rather than going through the email itself.
I make quite extensive use of it, so I support the creator, Leandro Facchinetti (who seems to have some other cool side-projects too) on Patreon—if you end up making use of this, you might want to also.
If you want to find some newsletters I subscribe to in this way, check out my blogroll page, which includes the (substantial) number of email newsletters I subscribe to!