Washing up liquid is a degreaser. While this sounds like a good thing for cleaning pipes, it can actually have a counterintuitive effect in a sewer system.
While pouring a few drops of washing up liquid into the toilet isn't going to destroy your plumbing instantly, it is bad plumbing hygiene. The risks of suds overflow, grease sludge buildup, and septic system disruption far outweigh the convenience. When it comes to the toilet, the rule is simple: Everything else, including dish soap, belongs elsewhere. washing up liquid down toilet
For routine maintenance, consider these safer and more effective options: Can I use washing up liquid to unblock toilet? - Smol Washing up liquid is a degreaser