Bitcoin Dominance (BTCD) is a ratio that measures the market capitalization of bitcoin (the largest digital asset by market cap) against that of all other cryptocurrencies, or altcoins. It's a ratio calculated by taking the market cap of BTC and dividing it by the market cap of all other altcoins.
And as More Altcoins are added to the "Space", so the Bitcoin Dominance Drops. As of Feb 2022, there were over 17,000 Altcoins ( Data from CoinMarketCap) and since then, an estimated 9000 more have entered, each time, BTC/d will Drop.
Also, Note that near BTC ATH, This Dominance Drops as money leaves BTC and enters the ALT coins Space. The declining drops show us that "ALT SEASON"s, post BTC ATH, have been Dropping in Volume. This is most likely to the simple fact that there are now a lot of HODL in Alt's
Is BTC'd Still a valid metric to use ? YES but only in a relative manner. Its % of market share will continue to drop as more ALTS enter the space ( And conversely, as Bad ALTS collapse, BTC will take it back maybe )
What Is still important to look at with BTC/D is the rise and Fall on the longer term time scales. This Will Show a longer term Trend and so hints towards new Bull runs etc
Shorter term time frames can pose problems when confused with moves associated with simply being oversold / overbought
However, if you look at MACD, RSI And BTC/D and see the associations between them, it is possible to pick up early signs of larger moves as Money Flows into Bitcoin from other sources.
For instance, It can be very interesting to view BTC'D alongside ETH.D and OTHERS.D and see the Flow of money