When you made the purchase, 1 year of Software Maintenance Service was included with the purchase. Up until December 31, 2020, you can open support cases, and you are entitled to use any new MATLAB releases released up to that day.
After December 31, 2020, you will not be able to open technical support cases, and you will not be entitled to any new MATLAB releases after that date. You will still be entitled to use everything up to R2020b as long as you want, but your upgrade service and technical support rights will expire.
It is, however, possible to renew Software Maintenance, typically one year at a time, but up to three years at a time I think it is, to continue to have rights to new releases and rights to open support cases.
Your rights to use the existing software do not expire: your right to use future software expires.