赞同邹老师的分析。正如您所说，since this year 和since now 的性质是一样的。常见词典对since的解释为：from a particular time or event in the past until the present, or in that period of time.（朗文），since 后一般接过去的时间点或段。

我查了国外的一个英语论坛，解答人是这样说的：

“Since” can only be used (formally) when starting at a point in the past, that discounts the present. You can say “I’ve been doing since last Tuesday” or “I’ve been doing since 1999”, because those are both solidly in the past. However, “I’ve been doing since 2020” is currently incorrect, as 2020 includes the present moment.(此贴是2020年发的) You could also say “I’ve been doing since this spring” as it’s no longer spring now. Informally, people sometimes do use “since” to refer to times that start in “the present.”

https://ell.stackexchange.com/questions/255141/present-perfect-since-this-year-this-summer

关于since now 的用法,我查了COCA共123例子，但这里包括大量的since的连词用法，仅是since 和now比邻而已，如：

There is a lot of competition with other designers since now everyone is doing this type of art.

介词用法占少数。如：

1. Since when do you care? Since now.

2. Since now, some disk-scheduling algorithms have been proposed to schedule real-time disk requests.

甚至可以用since +将来时间

1. Since when do you cook? Since tomorrow.

2. Since tomorrow we won’t have a home anymore.

显然这些是在特定语境中的口语表达，应该属于非正式用法，虽然也不能说是错误。

因此，您提到的《大学英语深层语法》的例子列出since now, 也不能算错。如果是正式文体或考试，还是不要这样用，正如您所说，改用from now on的形式。