The functionality still exists in current version of Mathematica, but it has slightly changed for several times in past versions.
According to my test, the functionality is introduced in v3, but auto conversion doesn't happen in any version if one directly types the expression inside the notebook. In v3 and v4, one needs to manually convert the cell to StandardForm
by e.g. pressing Ctrl+Shift+n as the last step:

In v5, one more extra step is required. One needs to first create a cell and convert the cell to InputForm
by e.g. Ctrl+Shift+i, then enter the code and convert the cell back to StandardForm
.
Start from v6, raw InputForm
is introduced, and one needs to create a cell and convert the cell to raw InputForm
by e.g. Ctrl+Shift+r, then enter the code and convert the cell back to StandardForm
. This remains to be the behavior of v12:

In[1]
. The notebook cell is okay with the paste but somehow does not accept it character by character. I was wondering why. Thanks for your kind response. $\endgroup$