Up to what frequency can coaxial connectors be used?

That depends. Do not use PL connector for frequencies higher than 50 MHz, so it is incomprehensible why some equipment manufacturers use this type even for VHF. Use types like N, BNC, SMA, TNC etc. for very high frequencies, especially if the surface is refined (gold-plated, silver-plated, etc.) However, this often reduces the number of permitted mating cycles because the thin layer of precious metal wears off quite easily. A very durable and high-quality surface finish is that with an alloy of white bronze (Cu + Sn + Zn), trade name TresAlloy or Smart3-Alloy.