I forgot about this change, it is fixed, BUT still possible with the use of .flags variable (set 0x4 bit), but... I don't think Lua , PROP or the old commands can change this since it is usually for internal things.... (It is automatically set when loading old moving SPNG saves)

A current workaround is to pause, then set the velocity with console as normal, then immediately save (or cut/copy), upon reload (or paste the stamp), it should move.

If you want moving sponge, just make a stamp of it in an older version and open it in the new version, it will move (or do what cracker64 said). I don't understand why it was fixed though, the bug with gel moving it was fixed.
In my mod, I actually turned the glitch into a feature. It can drag along solids such as CLNE or PSCN with it. Maybe that could be added as a replacement.