This interesting - I Googled UK Mains Frequency and got some good info, including this blog post.
https://www.emcstandards.co.uk/don-t-rely-on-the-stability-of-the-mains-freque
This had a cautionary tale of what happened when much of the UK grid was brought down by some big lightning storms. The main thing to suffer was the grid frequency which dropped briefly below 49Hz. Siemens had designed new electric trains with TPWS systems (Google it) and apparently if the mains frequency dropped to 49Hz for some reason this could cause interference with the systems which are safety critical. So the software monitored the frequency and shut down the train controller, stopping the train, in a way that required someone to go to the train with a laptop to restart it. With the trains stopped all over the rail network this took some time.
So mains frequency is important and systems should be designed to take account of variations in it, preferably not by just switching off! And another site shows how the variability has increased with more renewable power as this doesn't rely on large rotating machines.
Apparently for political reasons (some countries not contributing their whack of power) the long-term average frequency in Europe is somewhat below 50Hz at the moment.
So I think the OP's project is right to include frequency monitoring!