Indirect speech is by far the main use of the Konjunktiv I in German (the distant second being recipe-like instructions) – so one could argue that if you do not use the Konjunktiv I here, do not bother learning it for active use at all. Using the Konjunktiv I for indirect speech in any situation will arguably understood by every native speaker and is certainly not wrong.
However, using the Indikativ instead is so common that the main reason to consider it wrong would be because the grammar book says so (see this question on what can be considered right and wrong in German grammar). So if you use the Indikativ for indirect speech at almost any occasion, most people won’t even notice.
Note that, in German, there is no special grammar for certain formality levels or types of communication¹. People are just less sloppy and language change arrives later in written communication, higher levels and formal occasions (as it probably is the case with every language).
¹ In constrast, e.g., to French (in particular regarding the subjonctif) – at least according to what I was taught in school.