Does downshifting semiautomatic gears to slow the car down cause undue wear to the transmission?

The automatic transmission has inbuilt protection features that will not allow you to downshift if it would exceed the engine’s maximum RPM or exceed the abilities of the transmission. Most vehicles will also now “rev match” the change by disengaging the transmission, lifting the revs and then reengaging the transmission. If you are driving sensibly, then this style of active driving would be of no harm.