each search operator is allowed to treat field value case sensitivity as it sees fit.
search is case insensitive stats is case sensitive sort is case sensitive
Field values are not case sensitive. When searching for plain text tokens like foo, and phrase searches like "foo bar", these are are not case sensitive either.
On the other hand field names are always case sensitive, in the search command and in other commands.
eg if you have a field extracted as 'myfield', searching for myfield="bar" will work, whereas myField="bar" will not.
As far as other commands besides search, arguments and values are generally case sensitive and my advice is to assume that they are until proven otherwise.
I believe this picture was the same back in 3.X but im not positive.
Also, by default, values in lookup tables are case sensitive but you can change them to be case insensitive in transforms.conf.
Check out transforms.conf.spec in $SPLUNK_HOME/etc/system/README
* If set to false, case insensitive matching will be performed for all fields in a lookup
* Defaults to true (case sensitive matching)