I'm not sure about if it would be included - Snapshot images, for example, don't. However, currently luCi uses self-signed certs for things, although I'm sure you could setup a LE-Acme system if you felt the need to (I don't see a reason for it since self-signed is just as good as a LE cert, just warns it's self-signed.)
As others pointed out before, the default behaviour will be a self signed certificate.
I've installed luci-app-acme and acme-dnsapi (or whatever it's called) to generate a cert for openwrt.example.com (ofc my own domain), and setup DNS redirection that points to my 192.168.. router IP.
Luci SSL is listed as "optional" in the release goals for 21.02, not sure what that implies (since as of now there is only 1 image per device? Are they gonna come up with two separate no-ssl image and ssl image?)
Most likely, it won't come with luCi at all, and you'll install it via opkg update && opkg install luci-ssl if you want it after installation, or bundle it in an image builder if you are going that route.
I've never actually installed a Release image.. All my images have been either Snapshots or self-built from the tree.. Interesting that luCi is included by default, but makes sense from a general user standpoint.