I think you mean the 'subjunctive', not the 'subjective' ? 

No, the near future of something in the past, using (imparfait of) aller-infinitive, is not the same as the subjunctive mood, and is not expressing doubt or a subjective statement (on its own).

Using imparfait in near future 'tense' describes something that was definitely 'going to be done' (but was interrupted, stopped by something else etc), or that did still 'get done'. That is what the speaker/writer is conveying when they use this construct.

If you want more detail on the use of the subjunctive - a big topic, there are lessons on the site here, or many other detailed explanations to be found online :

https://www.lawlessfrench.com/grammar/subjunctive/