Account system is deprecated. Now, how can we set a "from account" when using the "sendmany" function?
The problem is, for instance, when I call sendmany method to send BTC 1 out of my service node, the asset distributed in the user addresses of my service would be randomly reducedand the total reduction volume should be BTC 1. In other words, I cannot extract the BTC 1 certainly from the targeted user address whose owner should have used our service to pay for something worth BTC 1, and the burden of BTC outflow shall be shared for all.


It never did this in the first place. The "from account" never did what you described; it did not select coins from addresses belonging to that account. Rather what it did was it would select coins from the entire wallet (so "from" all accounts) and the resulting transaction would say that it was reducing the "from account"'s balance. At no point in time did the "from account" parameter effect what coins were being selected.

If you truly want user address separation, you can use the multiwallet feature where each user has their own wallet file with its own keypool, private keys, transactions, etc.