"As of January 1, 2020, Medigap plans sold to new people with Medicare aren't allowed to cover the Part B deductible. Because of this, Plans C and F are not available to people new to Medicare starting on January 1, 2020. If you already have either of these 2 plans (or the high deductible version of Plan F) or are covered by one of these plans before January 1, 2020, you’ll be able to keep your plan. If you were eligible for Medicare before January 1, 2020, but not yet enrolled, you may be able to buy one of these plans."
From:
How to compare Medigap policies | Medicare
A chart on the site compares the plans and what they offer. This feature appears to be the only difference between F and G.
I have always had Plan F and plan to keep it for now. I suppose it may be phased out at some point.