There's much confusion about this... from what I understand:
1. If you need hyper-v features enabled and running concurrently with Workstation you need both WS version at least 15.5.5 & Win10 at least 2004 (aka 20H1)
2. If you don't need the hyper-v features you must disable them to run any version of WS earlier than 15.5.5
The above are necessary but not sufficient to guarantee 100% that someone with 1809/1903 will be able to run WS 15.5.5, therefore occasional weird cases appear in the WS Community and elsewhere. If you have available a second PC for experimenting, trial and error is the best strategy. If not, make restore points, backup VMs and try on your main PC, ensuring that you uninstall your current WS version completely before trying a new one and taking into account that the WS installers don't support downgrading very well.
The release notes are a bit vague and imho are a bit biased towards the needs and aspirations of people needing the latest and greatest stuff from Workstation together with the ever-changing bar of features/requirements set by Microsoft after each update. Beginners to virtualisation and users decided to stick with what works robustly are (and should be) hesitant to upgrade a system that just works. Therefore the questions like yours are totally valid, non-trivial and should be answered before experimenting with upgrades, especially given that each PC is its own mini-universe with its own maleficent ghost living inside©
I am afraid there are too many variables involved and somebody should compile a multidimensional compatibility matrix along several axis:
Win10 Home ---- Win10 Pro+
Win Versions <2004 ---- Win Versions >2004
Hyper-V enabled ---- Hyper-V disabled
Workstation ---- Player
WS/P <15.5.5 ---- WS/P >15.5.5
