Scriptlet is a javascript function that provides extended capabilities for content blocking. Scriptlets are necessary because native Safari filters are very limited in scope and cannot block everything.
Both adguard and ublock origin relies heavily on the use of scriplets. The scriptlets supported by them are available in their respective repositories - https://github.com/AdguardTeam/Scriptlets and https://github.com/gorhill/uBlock/wiki/Resources-Library
Adguard filters, IMHO best filters there are currently, has significant number of scriptlets (eg. base, annoyances, etc.). They can be searched using +js
.
Brave browser also supported uBO's scriptlets in its native adblock engine - https://github.com/brave/adblock-rust, https://github.com/brave/adblock-resources.