Is using software that unblocks websites and programs blocked by the telecommunications company, or helping others to do so, considered a sin? And is using websites and programs before they were blocked, and then their subsequent blocking, considered a sin?

If the blocking of websites or programs is for a legitimate interest recognized by the telecommunications company or government agencies, then it is not permissible to circumvent it to access them. However, if the blocking has no valid purpose, then there is no harm in accessing and benefiting from them. And whoever used these sites before they were blocked is not at fault.