Chemical Formulae and the Periodic Table of Elements
According to this entry in the IUPAC Gold book

https://goldbook.iupac.org/html/D/D01648.html

the symbol for Deuterium is H together with the isotopes notation. 

However deuterium and tritium are routinely indicated by D and T respectively. Their use is dictated by the framework. Besides nuclear reactions, when only straightforward chemical properties* are considered, it is convenient to use a single letter rather than H and super/sub-scripts.

In organic nomenclature, while writing names for molecules, the presence of deuterium is signalled by the symbol (d).

*note that isotopic effect might however affect chemical properties at fine levels. The toxicity of heavy water is a notable striking example.