SLIP39 or Secret Shamir Sharing is made by same Trezor developers who made widely used BIP39 format.
They made some improvements with changing the list of words and their number to 20, instead of standard 12/24 that is used for BIP39.
There are 1024 words in SLIP39
WordList and you can find it released on trezor github.
I won't get technical here but you can find more information about SLIP39 on
github and
trezor website.
You can use SLIP39 as a single backup with 20 words, or as multi backup that makes them much safer and harder to lose.
Bitcoin wallets currently supporting SLIP39:
I am focusing only on Bitcoin wallets here, but there are also some altcoin ethereum wallets that support SLIP39 format.
If you know for sure any other siftware and hardware wallet that support SLIP39 please report in here.