Example clarification n'avoir ... pas and ne ... plus depuis

Question

To emphasise that a (recurring) action in the past has now stopped happening with depuis, you can also use Présent indicatif with ne ... plus (not any more) instead of ne ... pas. Here ne...plus focuses on the change between the past situation and the new current one, which it highlights, hence Le Présent.

Tu ne bois plus d'alcool depuis cinq ans.You haven't drunk alcohol for five years.Je ne fume plus depuis 1998.I haven't smoked since 1998.
I am confused about these examples. I understand the structure and they seem to be more or less interchangeable, but I want to understand the difference. The qualifier makes sense, to indicate that the action has now stopped, but the examples don't seem to illustrate that.
How do those English sentences indicate that an action has now stopped occurring? "I haven't drunk alcohol for five years" -- termination began five years ago when I stopped drinking. Does it mean that the term of the five years has just completed?
But then, if so, with "je ne fume plus depuis 1998," we don't even have a defined term, it's that year to the assumed present and the stopping smoking happened in 1998.
I really want to understand so thanks in advance for any clarification!

Answer

a) Tu ne bois plus d'alcool depuis cinq ans (You haven’t drunk alcohol for 5 years)

b) Je ne fume plus depuis 1998 (I haven’t smoked since 1998)

For both, the English translations are free rather than literal with the change in tense. If you use more literal translations keeping the present tense of French:

a) You are not drinking anymore (any longer) alcohol since 5 years

b) I am not smoking anymore (any longer) since 1998

I am not sure if you meant to include an example with passé composé and ne . . pas for comparison, or it is that one example uses a date (time point) and the other a duration for ‘time’ that you are asking about?

French

1. When «depuis» is used it refers to something that is still ‘happening’, (except in specific circumstances - see link below) and started when the time period indicator states - either from the time point or the start of the duration indicated

2. Regardless of whether the start point is given by time point or duration the action has been happening 'since'. 3. When using ne. . plus and depuis - the emphasis is on the ‘happening’, which with the negation is 'not doing something anymore (any longer)’ (gets confusing right around here, the event is a non-event, so to speak!)

English

1. Indicates the action has been happening 'for' a duration OR the time ‘since’ it began. 2. We can use a similar format to ‘ne..plus’ in English to emphasise how long it is since we stopped doing something, as shown in literal (and clumsy) translations above.

In French the fact that the ’not doing’ is still occurring is made explicit by using depuis and the present tense. In the usual way of saying these phrases in English, the meaning is implicit as there is no caveat attached to the statement.

https://www.thoughtco.com/depuis-pendant-pour-1368831

Hope this helps.