[Radio1950]

There seems to be some unnecessary concerns.

Use of a coaxial “T” as a test port in normal low power HF RF measurements.
Use of a 10x CRO Probe in HF RF measurements.

In practice, in the home workshop, within normal common sense restraints, within manufacturer’s specs, and in the context of the original request by the OP, there is nothing wrong in using either.

I set up a test.

1 - I fed my ICOM 7600 TRX on 3.5 MHz, ATU off, FM, 50W, no mod, into my HF Power Meter on the 50 W FWD scale and terminated with a 50 ohm load.
2 - I read the reflected power on the 2w scale as almost zero.
3 - I place a good BNC “T” at the load input, and repeat, with no visible change to the reflected power.
4 - I place the tip of an "economy", compensated, 10x CRO probe on the “open” end of the coaxial “T”, with probe attached to a 20 MHz CRO, and observe just the very slightest change to reflected power, as expected.
5 - I repeat all at 28 MHz, with just a slight more change to the reflected power, as expected.
6 - I repeat all with a UHF coaxial “T”, with no change to results of previous steps.

Now, I wonder if someone here, with more accurate test equipment, can test the changes as above with a BNC “T” , and report back here in terms of both SWR and Return Loss.

Hands up all those here, with careers in RF, who have never used a BNC “T” as a temporary cable joiner at video or HF?
I have one hand ready.