For low power devices, they are probably safe even if all the power is reflected. If you generate more power than you need then feed the output through an attenuator it will reduce the effects of any reflected power.
For example, a 6dB attenuation will reduce the forward power by a factor of 4, but any reflected power will be reduced by an additional factor of 4 on the return trip, which gives a factor of 16 reduction in any reflected power.
These effects can normally be ignored on low-power devices (VCOs, PLLs, etc) that can tolerate operating into a mismatched load with no ill-effects. They don’t need any special protection.
For special-purpose devices, the datasheet will often have limits or recommendations to protect them.
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