Jmods said through a live stream that RS3 is undergoing a deflation, contrary to many people believe:
"Well, I'm just paraphrasing Suity's TL;DW from the recent Q&A livestream, which says:
We were in a period of deflation last year which is the complete opposite of what people think.
Inflation on big ticket items like partyhats is completely artificial and not due to inflation in RS"
https://www.reddit.com/r/runescape/comments/8cof0k/only_jagex_would_say_that_gold_isnt_suffering/
There is no mistake. More gp keeps coming into the game but from all sources, noticeably bossing, slaying and such. However, nobody seems to be in a frenzy to spend any surplus gp so there is only deflation but not inflation.
When nobody cares about spending, wasting, dumping their gp, there is really not a need for more or less gold sinks because those sinks don't mean much to the economy.