We don't have clear guidance on whether keywords and identifiers should be uppercased or not.
General perception is that we should lowercase keywords and put identifiers in camel case. This is the prevalent formatting in most modern programming languages and commonly perceived as concise and readable style.
data maxWaitTimeInSeconds type i.One of the reasons behind this is that the human eye parses mixed and lower case faster than upper case, probably due to the more varied and thus easier to distinguish shapes of the letters.
However, ABAP is case-insensitive, such that camel-casing identifiers remains a mere optical improvement that gets lost when formatting and moving the code to the database.
MAXWAITTIMEINSECONDSIn response to this, ABAPers decided decades ago to prefer snake case to camel case, which at least preserves the word borders.
max_wait_time_in_seconds
MAX_WAIT_TIME_IN_SECONDSAt the same time, they decided to uppercase ABAP keywords to make them easier to spot in black-and-white printouts and on monochrome displays.
DATA max_wait_time_in_seconds TYPE i.Even though large color screens invalidated part of these arguments, the keywords-uppercase-identifiers-lowercase format turned into a de facto standard that rules a vast majority of ABAP's global code base.
The section Case in the ABAP Programming Guidelines follows this de facto standard.
The section Using Pretty Printer in the ABAP Programming Guidelines contradicts and says the recommendation should actually be lowercase keywords and uppercase identifiers. We do see foundation code that prefers lowercased keywords.
In summary, we can only recommend teams to discuss this and agree on one common style.