I think you're missing that the feature we're talking about is specifically within the $5/month option, in much the same way that ProtonMail (the one I use), also hides their catch-all option behind a higher price-point.
I'm also fairly sure that they don't let you reply as the address the catch-all was placed on, which is an important feature.
True, I missed that "custom domain" isn't part of the lowest tier.
The point still stands though, if you have a mail provider that supports custom domains there usually isn't a limit on the catch all aliases you can add (or a very high one). In any case it's far from being "prohibitively" expensive.
> I'm also fairly sure that they don't let you reply as the address the catch-all was placed on
That is definitely possible with Fastmail as that's how I use it. In their case I believe that feature is called "Alias". If you reply to an email you just select with alias you want to reply from.
I use Cloudflare Email Routing. Only problem I currently have is if I don't think ahead, I might have to setup the alias on the spot (probably should setup a catch all to make sure that it doesn't just drop them). And that redirects stuff to generally anywhere (though I mostly have it setup to redirect to an outlook.com family plan, which is already paid for for other reasons).
I do something similar with the old Gmail for Domains product (or whatever it was called before Workspace) - it lets me add real mailboxes but also have a catch-all, where everything else gets delivered to a specific address.
I know this is not available any more so I've done the same with Cloudflare Email Routing which lets you set up a catch-all and is (still) free.
