I don't think that stats indicates true picture. Away tours had always been difficult and it's more or less same always - and I must say, I don't mind home team putting everything in their favor as long as it's not cheating to make it difficult for tourists - that's why it's called Test cricket. How can we put an end to one sided tours is a separate discussion, for which I can discuss later, but these set of data has to be interpreted with a bit of caution as well.
If we look at the charts, significant difference is the Draw % - less and less Tests are ending in draws these days, mainly for 3 reasons -
1. Match duration: technology and change in rules for make up time has made Test cricket 450 overs game, instead of 30 hours, when batsmen (of team in struggle) could change box every hour and umpires would call time at 5PM.
2. Globalization - good - bad I won't comment, but I was happy in 1980s & 90s when 6/7 countries were playing Test .... now we have 12.
3. T20 era - batsmen are less equipped to survive on challenging conditions & capacity to play spin has gone significantly low. These days, batsmen are very good shot makers on roads, but hardly possess the technique or temperament to bail out a difficult phase of play.
NOW, what happened in early days of that data set (80s/90s) is that tourists were still struggling on away tours, probably as much as now, but lots of Tests those days ended in draws because of lack of time, and majority of these drawn Tests, if we analyze game by game scorecards, we'll see was dominated by home teams - they just ran out of time. I give one example from PAK, which had a very good team in 1980s & 1990s - more or less, they dominated almost every Test at home barring one team in those 2 (3, if we take 1970s) decades, but 350-360 overs game duration wasn't just long enough to force a result. If I put the same contest in present time - every Test that PAK won in UAE on 5th day were destined to draw in 350-360 overs game, and majority dominated by PAK.
More or less, this was the case for other teams as well - IND had 4 Series in late 1970s of 6 Tests (WI, PAK, AUS, ENG), and they won all 4 series, winning 6 Tests out of 24 & rest 18 draws ......... apply same rule of current days - that's 90 overs/day, if required 30 minutes extension, severe fine and demerit points for deliberate time wasting, up to 1 hour extra make up time for weather interruptions and use of flood light if required ...... my hunch is those 24 Tests would have ended like 18-20 direct results and 6-4 draws and obviously IND would have won most of those 18-20 even then. In fact, they drew the Madras Test against SRL in 1981.
Another factor was in 1980s & 1990s, till early 2000s, we had 2 outstanding teams that often took H/A factor out of equation, which isn't happening now. If we had just one team who could compete equally (not dominate like the 2) in 3 different conditions - IND, AUS & ENG, this stats would have been at least 3-5% closer over all. I can safely say, had current 450 overs rule were applied, we could have counted drawn Tests involving that WINs in one hand over the entire decade ..... and WIN would have won most of those Tests, be it in AUS, ENG, IND or at home.
In that regard, hypothetically, if we attribute a direct winner among the close draws in 1980s, to bring that Draw % to like 23% (simple words - those Tests being drawn for less playing time, put a possible winner for half of the draws, to bring it like 23% draw), I am sure that home % will reach close to 50% even in 1980s, despite one team winning 5-0/4-0 in ENG, 3-0 in IND and 4-1 in AUS. There are not many teams in history, who could challenge that status co and it worthy enough to wait for couple of decades to see one such team, if so.
