microstrip line at frequency below 100MHz

Is there a lower cutoff frequency for microstrip lines, similarly as waveguides do have? Can they still be viewed as transmission lines with characteristic impedance Z0, at such low frequency?

at 100Mhz you typical need a very long line for your PCB trace to qualify as a transmission line unless you have a ridiculous dialectical. For example in FR4 ER=4.9 @ 100Mhz a quarter wave length is 13.339inch or 338.82mm. Most folks consider 1/10 the wave length a transmission line. If you keep the lines under 5inchs I would not worry about it to much.

Coaxial cables and microstrip lines are operating in TEM mode, which involves no cut-off frequency.

Z0 exists for all frequencies, but as cmontoya mentioned, you possibly don't see much transmission line specfic effects.

Thanks for answers! I hope this will not have any big influence on measurements in case device under test will be connected through coax cables>SMA>short microstrip lines

For 100M signal, if you want match very accurate, you can use tools for the 100M trace width, and you will find the trace is too wide to accept.
But you can add PAD at both end to improve VSWR.