The use of T for zero, N for nine, etc. is known as using “cut numbers”. Cut numbers are extremely common in amateur radio CW, especially in contests for the RST and serial number. E.g. ENN TTA is unambiguously 599 001. Most competent CW operators will have no difficulty with cut numbers used in this fashion.
The most important rule is that cut numbers are ONLY used when the context makes it unambiguously obvious that it is a number that is being sent. So cut numbers are never used in callsigns, for example. Using cut numbers in SOTA references is OK, because the format of a SOTA reference always has two letters followed by three numbers.
T for Zero and N for 9 are commonly used.
E for 5 and A for 1 rather less so.
The other cut numbers are almost never used because they offer little benefit.
73,
John, G3WGV