0.5% right now sounds about right.
I got into Dominion during S3. Found it a lot more fun than SR. Honestly hate a lot of the "PvE" aspect of SR. And I enjoy the team aspect of Dom and ARAM.
Back in this time there was an active Dominion community. We had people hosting weekly tournaments. We had our own section of the old forums to help newer players along.
Between ARAM coming out and the loss of the community/tournaments(guy hosting left). Dominion was set to fail.
It got most of the bots(I assume this is why they left it for so long, it kept SR free of most bots). New players had an extremely hard time getting to enjoy any factor of the mode because of this.
It never got any attention to fixing it's queues. We said ranked queue would only require flipping a "switch". We asked about giving us Blind Draft over Blind Pick. Was told they'd look into it. No work about that after. Having 6 bans would of greatly help the "OP" feel of the mode for some champions. Mostly revolving around bot lane champions.
Over the past ~2.5 years I've played Dominion off and on, I've seen very little assistance to it. We had a short time with ManWolfAxeBoss. Who did a lot of good in balancing parts of it. But his time on the mode was short.
All of this over that time frame. Loss of community, bots making up a lot of the lower MMR matches and lack of a good queue. Of course it would have such a small number of people in it. It was never given a serious chance to start off with. It had bugs when it came out that made it bad to play. And nothing was done to fix the mode after that. You just let it die off.
I'd love to numbers over the past few years. I'm sure we can see when it had drop offs and why.