Glossary
Mask
An extension for clicking custom content elements together in the backend. No longer developed for TYPO3 v14.
What Mask achieved
Custom content elements are the normal case in TYPO3; hardly any project gets by with the ones that ship. Classically they were assembled from several parts in different places: TCA for the fields, TypoScript for the rendering, a Fluid template for the output.
Mask brought that together into one interface: create fields, name them, point at a template, done. For teams without their own development capacity that was a real gain.
Where things stand
Mask is not being developed for TYPO3 v14. Content Blocks takes its place; it arrived in the core with v14 and pursues the same basic idea of keeping everything belonging to a content element in one place.
For existing projects there is a migration path. It does not take the mapping off your hands, but it means nobody has to rebuild every element by hand.
What that means in practice
If you use Mask and run a supported version up to v13, there is nothing urgent to do. If you are planning the jump to v14, plan the conversion into the same stretch of work; the elements have to be checked anyway.
And anyone building new today takes Content Blocks straight away. The decision has been made; making it again only costs money.