Disney initially used a sticker program for unlimited soda refills. Quite some time ago they went to the refillable plastic mugs and continue to use them.
The reason that some recent cruisers may have been given stickers instead of the mugs can be traced to the virus outbreaks in the industry. Disney probably felt that the mugs represented an easy way to cut down on person-to-person contamination and did away with them for that reason.
My guess is that the mugs will return without notice and probably in the near future, if they haven't already.