These days we call it text messaging. In the 19th century it was emblematic poetry, but the two things are very similar. A poem called An Essay to Miss Catherine Jay, which dates back to the 1840s, reads:
He says he loves U 2 X S,
U R virtuous and Y's,
In X L N C U X L
All others in his i's.
There is also this elegy (LEG) to Miss Emily Kay (MLE K):
Sad nymphs of UL, U have much to cry for,
Sweet MLE K U never more shall C!
O SX maids! come hither, and VU,
With tearful I this MT LEG.
The British Library recently announced details of its exhibition later in the year devoted to the English language, and it will be exhibiting examples such as the poems above. More on the exhibition here. The second poem above comes from the Visual Thesaurus.