You may ask yourself, what does it mean to be a dead-but-learnable language? It must have numerous texts written in it and it must have an adequately researched pronunciation system. That's all I can think of off the top of my head since I'm distracted by The Big Bang Theory, so if anyone wants to add any other criterion, please do so.
Learnable:
Coptic
Old Babylonian, Middle, New, Late; Standard Babylonian
Old Assyrian -> Middle Assyrian -> New Assyrian
Eblaite (?)
Classical Arabic
Ge'ez
Classical Syriac
Biblical/Classical Hebrew**
Ancient Greek/Koine Greek**
Latin* **
- Old Galician/Old Portuguese
Old Leonese
Old Castilian
Old Aragonese
Old Occitan/Provençal
Old Catalan (splits from Occitan at some point)
Old Welsh -> Middle Welsh
Middle Cornish
Old Irish -> Middle Irish
Old Norse (Old Icelandic)
Old English -> Middle English
Old High German -> Middle High German
Old Church Slavonic/ Old Bulgarian**
Sanskrit**
Middle Persian
Pali/Prakrit
Avestan
Classical Tibetan
Classical Japanese
Old Tamil -> Middle Tamil
Ottoman Turkish
Chagatai
Classical Nahuatl
Classical Chinese
On the fence:
Sumerian
Middle Egyptian (no vowels)
Old Akkadian (pre sargonic -> sargonic -> Ur III) (???)
Ugaritic
Umbrian
Oscan
Gothic*
Yola*
Old Prussian*
Old Persian
Sogdian
Hittite
Tocharian A/B
Gaulish*
Norn*
Not-learnable:
Etruscan
Lepontic
Galatian
Noric
Primitive Irish
Celtiberian, Astur, Galaic, etc *
Lusitanian
Tartessian
Iberian
Phoenician
Moabite
Elamite
Hurrian/Urartian
Hattic
Phrygian
Ammonite
North Picene
South Picene
Venetic
Messapian
Lydian
Lycian
Sidetic
Pisidian
Carian
Illyrian
Dacian
Thracian
Volscian
Meroitic
Vandalic*
Lombardic
Burgundian*
Goguryeo
Buyeo
Silla
Baekje
Gaya
*reconstruction and/or revival movements exist
**in use as a ritual language