The abolition of Junior NISA was included as part of the 2020 tax reform, and while there is always the theoretical possibility it will be revived, the government's current plan makes no mention of it, nor has there been any indication from any other quarter that it will be continued beyond 2023. So unless something quite unexpected takes place -- not that that's absolutely impossible -- next year will be the last year for opening or adding to Junior NISA accounts. Once all pre-2024 accounts have run their course, under-18s will simply be out of luck.
As for Regular and Tsumitate NISA, only the outlines as described in the above posts have been made available. The decision made in yesterday's cabinet meeting simply formalized the plan that was announced on November 25 (which is why media coverage of the meeting was negligible), resulting in publication of the government's full 18-page
"Plan to Double Income from Financial Assets". This plan is supported by seven "pillars," the first of which involves NISA and runs to almost four pages in the document but offers very little in the way of specifics. Basically, as others have mentioned, the originally proposed two-level "New NISA" is to be scrapped in favor of something simpler that allows both Regular NISA and Tsumitate NISA accounts to run in perpetuity and to be increased in size. That's really about as far as it goes. Concrete proposals are to be considered and formulated by the Tax Advisory Committee from the middle of next month to around the end of the year, but it should be noted that this committee has already indicated some resistance to the announced NISA proposals as disproportionately favoring the wealthy. For now, we're still mostly in a wait-and-see situation (except regarding Junior NISA).
Cabinet-meeting decisions have legal standing as official administration policy, but they most definitely do not have the status of law -- that comes only through approval by the Diet, and it's never really over until it's over. By the way, actions taken at cabinet meetings are made public on
the prime minister's website.
For more details about how current Junior NISA accounts will be handled in 2023 and 2024, see
the official explanation on
this page or the less bureaucratic explanations
here or
here.