Flight plans for flying in G Class Airspace

A few years ago I flew in Portugal and it was required there.

Conversely, if one files a VFR Class G flight plan in the UK it is, in my experience, totally ignored and lost by the fragmented privatised ATC system. Flying across Belgium, Netherlands and Poland, my flight plans have always been known and used by ATC.

In Slovakia we don't have to fill FPL for G Class airspace, we don't need to fill Flight Plan even for international flights to Poland and Czech republic (Class G and Class E airspace up to FL095), for Austria it is a little bit tricky (you even don't need to fill FPL for arrival to Austria, but necessary to fill FPL for departure from Austria).
But, on other side we often do it mainly for long range flights and more comfortable crossing of different temporary restricted or segregated parts of airspace. If ATC, better say FIC dispatcher (Flight Information Center) is aware of flight details, it is less complicated to obtain proper information for your flight path.

For VFR entry into Austrian airspace you may need an flight plan, depending on the departure point. For IFR Flights to LOWZ, LOAN,LOAV, LOIJ wich ending in aispace G you need to file a flight plan.
Details see AIP-Austria
https://eaip.austrocontrol.at/

Many countries in Europe need a flight plan for G or even for every flight e.g. Greece.