Currently British passport holders can enter visa free to Montenegro for up to 90 days. And since Montenegro isn't part of the EU this status shouldn't be affected by Brexit. However please be aware that your passport should have 3 months validity on it after the planned date of departure from Montenegro and that Montenegrin authorities may not recognise any period of passport validity over and above 10 years (the UK passport authority is quite unique in the world in that if you renew your passport before it expires, any remaining time on it, up to a maximum of about 9 months, is transferred to the new passport - it is this extra time that may not be recognised by Montenegro).
All this information is subject to change and for the most up to date information please visit the relevant Foreign Office webpage, in this case:https://www.gov.uk/foreign-travel-advic ... quirements
I realise that my answer is more complex than a simple yes or no (more of a "yes, but...") but the main take-home message should be that nothing has changed and should not in the foreseeable future.