From Houdini 19
How much longer will Mantra be supported?

Development on Mantra is limited to critical bug fixes. The plan is for this level of support to continue until the end of 2024 (two years after Karma XPU is released) at which time Karma will take over as Houdini’s only supported built-in renderer. For as long as they are on AUP, customers will be able to keep their unsupported existing licenses until at least 2025.