Having an RCD up-front above 30mA is usually a TT thing, or sometimes a fire safety for agriculture sites. 100mA is more common for domestic, but 300mA is just as acceptable provided the earth rod is below 167 ohms.
I don't think there ever has been a requirement on 30mA / 100mA / 300mA (or possibly more) as such. The main point has always been to meet ADS times and for TT that is often only possible via an RCD as you will struggle to get a rod arrangement low enough to trip even a 6A B MCB in the required times.
I suspect 100mA is the 'sweet spot' as less likely to nuisance trip than 30mA for a whole installation, and can had delay so it is selective with downstream 30mA 'additional protection' RCDs, whereas 300mA starts to demand an earth rod impedance that can be a challenge to meet all-year in some soil conditions without quite an effort for several deep rods, etc. Or the no-no of relying on a nice long buried metal water pipe...