I heard that Barry owns half the team, if true, would explain why he's still around

Article says they might be losing Coca Cola too

What the fuck is going on

“People are making commercial decisions because we're fully aware that there's a lot more to the story, and if things do come out, they are potentially damaging to other people's brands.”

"The sport should be ashamed. They know the full story and as per usual side step around everything and don't say anything hoping the storm will settle like they always have. No protection for drivers ever."

So after some digging, asking a few people in the sport, here's what I believe has happened (all unfounded, word of mouth, but better rumour mongering than a facebook comment section).

Betty Klimenko fired Kostecki during the off season (I was told it wasn't Barry Ryan, but he still could have been involved in some way). The wording I was given was "biblical falling out".
They've tried to make up with him and re-hired him or reneged on sacking him.
He's realised that he doesn't want to be part of the toxic environment, having been stepped all over, so he's trying to walk. Erebus are trying to hold him to a contract he was essentially fired from.
Erebus have treated him like garbage, but Supercars have not shown any real support to Kostecki, hence a lot of people outside the sport (McLaughlin, SVG et al) have commented about his treatment.

This apparently also happened to SVG when Stone Brothers turned into Erebus.

But I don't get why both their major sponsors would jump ship over this, unless they want to follow Brodie wherever he ends up. Driver contract disputes rarely bleed into sponsorship termination unless the sponsor's relationship is with the driver or there's serious misconduct (like the kind that sponsors wouldn't want to be publicly associated with) involved.

This is not an issue of mental health, as alluded to by Supercars, at least not on the part of Brodie. This is a case of what appears to be a toxic workplace. Other teams and Supercars have seen evidence of this in the past. For instance, some 18 months ago an email (addressed to pretty much everyone involved in Gen3 development) emanated from the team in question that was so venomous that the CEO of Supercars was obliged to step in and attempt to smooth the waters. But long-term damage was done. That's just one example. There are others.

the commercial manager of Erebus is also, apparently, Brodie's manager. Brodie left, thus his agent left and took all the commercial agreements (sponsors) with him.

If Betty/Barry fired Brodie with that factor present, that is a whole new level of shooting yourself in the foot. Still seems like either the sponsors were committed for 24 but have pretty solid evidence of contract-breaking behaviour on Erebus's side, or Erebus are terrible at commercial contracts.