Hope you are doing well. I will try to answer both of your questions as accurate as I can.
1- The Digital signature certificate has always been a free tool. As far as my knowledge, this is available with freeware Acrobat Reader DC since the beginning. There isn't any discussion in pipeline about making this tool accessible with subscription. Can't commit for future.
2- There are frequent updates released for Acrobat/Reader DC on a certain time interval. Update doesn't affect the excisting functionality. It just brings new add-on to the existing features.