At my employer, we've been working on motor drive inverters and DC-DC converters running off 700VDC for a number of years.
We use differential probes for nearly all HV measurements and for looking at gate signals, etc. Scopes are grounded to earth via the normal AC power plugs.
In the early days of working on the 700V stuff, we didn't have much in the way for diff probes and I recall trying to get a look at the high side gate signals using a TEK THS720A scope. With isolated inputs, it was not rated for that sort of voltage but since it is battery powered, I figured just run off battery with only that 1 probe connected and use a wood stick to press the control buttons, etc. Basically, the scope was floated.
For reasons unclear, as the DC voltage increased, the gate waveform I was looking at became more and more distorted. Later was able go get a diff probe and that issue was not present.
Nearly 30yrs ago I worked at a plant that made power supplies (mostly off-line types). The line repair techs (repaired test failures) used to chop off the earth pin on their AC powered bench scopes and float the scope to whatever the internal DC link voltage on the power supply was when troubleshooting things. That could often be 200V. They got away with it but for sure was not a safe practice. That place at that time didn't really have any sort of safely rules or training....