Hi Vinisha Cv ,

there is no lower limit, you can use microstrips with DC (the GND plane is part of the microstrip, so there are two conductors forming the transmission line).

The upper limit depends on the PCB material and the requirements concerning signal integrity, posed by your application, as well as on the dimensions of the cross-section of the microstrip.

Regarding material:

For digital signals, you can use common FR4 easily up to 3 GHz.

For analog signals, some ceramic laminates work well even above 50 GHz. For example, have a look at

https://rogerscorp.com/advanced-connectivity-solutions/ro4000-series-laminates

Regarding dimensions:

You might have a look at the paragraph "Higher-order Modes and High Frequency Limitation" in the following text:

https://literature.cdn.keysight.com/litweb/pdf/rfde2005a/rfdemom/rfdemomA6.html

or, in more detail, here:

https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Microwave_and_RF_Design_II_-_Transmission_Lines_(Steer)/04%3A_Chapter_4/4.06%3A_Section_6-

In India power system frequency is 50 Hz. The maximum & minimum limit is 50.5 Hz & 49.5 Hz respectively in which a micro-strip transmission line can operate without distortion for a particular length and width of the trace

If you want to know the effect of microstrip on signal integrity, you need to consider bandwidth, not just frequency. Microstrip is dispersive, so all signals except CW will become spread out over distance.

See https://www.microwaves101.com/encyclopedias/dispersion for an introduction.