Relics is a Mindustry mod adding lost content from Mindustry Classic that was removed in updates. This is mainly turrets, as most other removed content has equivalents in the current game.
Solo, a modern version of the Turret from Mindustry Classic
Solo only shoots out of one barrel and has a slower fire rate than Duo, but it has a higher range.
Stream, a modern version of the Gattling Turret from Mindustry Classic
Stream fires bullets very fast and has good range, but shots are slightly more inaccurate.
Split, a modern version of the Splitter Turret from Mindustry Classic.
Split fires 7 bullets in a spread. It has high damage but limited range.
Incinerate, a modern version of the Plasma Turret from Mindustry Classic.
Incinerate fires a high-damage stream of fire, but has a low range.
Fragment, a modern version of the Flak Turret from Mindustry Classic.
Fragment fires 3 bullets that explode into flak. It has low accuracy but is good for large clumps of enemies.
Due to the large number of changes to Mindustry since v3.5, also known as Mindustry Classic, a lot had to be inferred about how these turrets would look today. There also have been changes to stat spread across the whole game, giving everything more overall health and damage, so those stats have also been inferred. Due to this, some turrets may need to be re-balanced in the future or might undergo changes. Some turrets have also been given a new place in progression due to changes to materials since v3.5. The stats are based on turrets that weren't deleted in the v4.0 and the general part of game progression you would unlock them in.
Although many things were deleted in v4.0, not everything was lost. Some turrets still live on today in new forms.
Turrets that won't be added because they already exist.
Double Turret already exists in the form of Duo
Railgun Turret already exists in the form of Hail
Flamer Turret already exists in the form of Scorch
Laser Turret already exists in the form of Lancer, albeit with slightly different mechanics
Tesla Turret already exists in the form of Arc