Why is this an issue?
Using upper case literal suffixes removes the potential ambiguity between "1" (digit 1) and "l" (letter el) for declaring literals.
Noncompliant code example
long long1 = 1l; // Noncompliant
float float1 = 1.0f; // Noncompliant
double double1 = 1.0d; // Noncompliant
long long1 = 1L;
float float1 = 1.0F;
double double1 = 1.0D;